In general, followings can be listed as factors of light transmission loss of optical fibers.
(1) Loss inherent in materials constituting optical fibers, such as Rayleigh scattering and infrared absorption loss.
(2) Scaterring loss due to fiber structure imperfection and glass flaws, such as scattering by irregularities of the interface of a core and a cladding, strias and bubbles.
(3) Absorption loss by impurities remained in fibers, such as absorption by iron and other transition metals, absorption by intermolecular vibration of hydroxyl group.
Silica optical fibers, which are currently in wide practical use, are free from losses caused by external factors mentioned in (2) and (3). When high purity silica glass is used as a core, it becomes possible to produce fibers with losses close to the theoretical limit of 0.15 dB/km. However, since the age of internet and other multimedia is coming, it is required to develop fibers with losses less than those of silica fibers, in order to reduce communication costs by increasing the repeater span and to enlarge communications networks, and studies save been constinued both inside and outside Japan.
In the stage of developing materials, it has been especially demanded to decrease Rayleigh scattering, which is the main cause of loss, and efforts have been made to find multi-component glass with minimal Rayleigh scattering. For example, Japanese Unexamined Patent Publication (KOKAI) No.105483/1993 discloses a large number of multi-component glasses which can lessen density fluctuations, which are the main cause of Rayleigh scattering. It has been expected that multi-component glasses can realize superior fibers than silica fibers, but, at present, none of the multi-component glasses are practically used as fibers for long distance communication.
The multi-component glasses have following disadvantages:
(1) Density fluctuations are increased and hence, light scattering losses are increased, in comparison with one-component glass.
(2) Degree of crystallinity is high and microcrystallization in spinning fibers is hard to be controlled, and therefore light transmittance is degraded in comparison with one-component glass.
(3) It is difficult to control impurities which absorb light with wavelengths used currently for communications, such as hydroxyl group and transition metals, and transmission losses are increased.
It is an object of the present invention to obviate the disadvantages of multi-component glasses and produce glass materials which can be produced into optical fibers with minimal losses, which cannot be realized by optical fibers formed of the conventional silica glasses, especially optical fibers which have superior Rayleigh scattering loss characteristics.